1. Field of the Invention
The present invention relates to a hardness tester and a method of testing hardness.
2. Description of Related Art
A conventionally known hardness tester measures hardness of a sample based on dimensions of an indentation formed by pressing an indenter against the sample with a predetermined test force. For example, a Vickers hardness tester presses a quadrangular pyramidal indenter into a surface of a sample and measures a length of a diagonal line of a formed indentation. The Vickers hardness tester calculates hardness based on the measured length of the diagonal line of the indentation.
In testing hardness of a metal material, a known hardness tester generally executes binarization by determining whether a luminance value is below a predetermined value (threshold value) with respect to an image of a surface of a sample captured by an image capturer, and then extracts an indentation area formed in the surface of the sample (refer to Japanese Patent No. 4029832, for example).
In the conventional hardness tester above, however, a change in luminance, such as shading, occurs throughout the captured image. Thus, the conventional hardness tester is sometimes unable to extract the indentation area accurately due to an impact of the change in luminance during binarization of the entire image. In the case of a small indentation area in particular, accurate extraction of the indentation area is difficult. In addition, the conventional hardness tester has difficulty extracting an indentation area accurately in a case where the vicinity of a formed indentation K2 is contaminated as shown in FIG. 14.